12 THE CENTRAL NERVOUS SYSTEM 



ence by excision leads to movements of the head (so-called head 

 nystagmus). The anterior extremity is represented by a centre in the 

 crus primum, and the posterior extremity by a centre in the cms 

 secundum, of the ansiform lobule of its own side, and injury in the 

 region of these centres is associated with abnormal movements of the 

 corresponding fore and hind foot respectively. Extirpation of a lobulus 

 paramedianus causes rolling movements of the body around its long 

 axis or bending of the body to one side, and this centre is connected 

 with the muscles of the trunk. It is still an open question whether in 

 the function of these centres only the cortex of the lobules is concerned, 

 or in addition the corresponding portions of the central nuclei of the 

 cerebellum. These observations are supported by other facts. For 

 example, microscopical studies have shown that de'finite regions of the 

 cerebellar cortex are especially connected with, definite levels of the 

 spinal cord. Further, the lobulation of the cerebellum in mammals 

 keeps pace with the increase in complexity of the voluntary motor 

 apparatus of the whole bod}'', and the variations in the degree of 

 development of definite lobules are related to the variations in the 

 anatomical and physiological development of the corresponding groups 

 of muscles. All this fits in well with the idea that the cerebellum is a 

 great reflex mechanism standing in intimate relation on the one hand 

 to numerous afferent paths (skin, muscles, labyrinth, etc.), and on the 

 other to the voluntary muscles. It is the precise nature of the influ- 

 ence exerted by it upon the latter which is in doubt, whether an aug- 

 menting sthenic influence, as Luciani supposes, or a co-ordinating in- 

 fluence, as Flourens assumed, or a combination of these. 



Forced Movements. We have incidentally mentioned that in fishes 

 injuries to the semicircular canals may give rise to movements which 

 seem to be beyond the control of the animal, and which have conse- 

 quently received the name of ' forced movements.' It may be added 

 that when the internal ear of a Necturus (one of the tailed amphibia) 

 is destroyed on one side, rapid movements of rotation around a longi- 

 tudinal axis are observed. The animal spins round and round ap- 

 parently without voluntary control, purpose, or fatigue. The direction 

 of rotation is towards the side of the lesion, the observer being sup- 

 posed to look down upon the animal as it lies in its normal position. 

 After a time it becomes quiescent; but the forced movements can be 

 again produced by pinching or exciting it in other ways. In man, too, 

 during the passage of a galvanic current through the head by electrodes 

 applied just behind the ears, a tendency to move the head towards the 

 anode is experienced. The person may resist the tendency, but if the 

 current be strong enough his resistance will be overcome; he will exe- 

 cute a forced movement. When the head turns towards the anode 

 the eyes move in the same direction, and then undergo jerking move- 

 ments towards the kathode. There is at the same time a feeling of 

 vertigo. Complex as such an experiment is, involving as it does stimu- 

 lation of so many structures within the cranium, there is reason to 

 believe that it is the excitation of the semicircular canals, or their 

 cerebellar connections, that is responsible for these forced movements. 

 For when the experiment is performed on a pigeon, forced movements 

 are caused so long as the membranous canals are intact, but not after 

 they have been destroyed (Ewald). The observation of Rawitz, that 

 the peculiar rotatory movements of the so-called Japanese dancing 

 mice are associated with marked anatomical peculiarities in the laby- 

 rinth, is another fact in favour of the connection of the canals with 

 the maintenance of equilibrium and the sense of rotation. So is the 

 relation between the degree of development of the canals in different - 



